(a) Field of the Invention
The present invention relates to a process for preparing a fuel pellet for nuclear reactor.
(b) Description of the Prior Art
The prior main process for preparing a fuel pellet for nuclear reactor is to make a powder of oxide, carbide or nitride having the same chemical form as the fuel by any means and press mold the power and sinter it. In this case, the contamination of manufacturing institution by finely divided powder is a problem because of handling powder. In particular, in case of handling plutonium, or in case of handling a recycling fuel in thorium cycle, as a substitute method for avoiding the handling of powder, pellet manufacturing method by the above described "gel particle pressing method" which is called "Sphere-Cal" [Literature 1, see below], "COGEPEL" (COnversion using GElation to produce PELlets) [Literature 2] or "SGMP" (Sol-Gel Microsphere Pellatization) [Literature 3], using a gel particle manufactured by a sol-gel method developed in the field of manufacturing of high-temperature gas-cooled reactor fuel sphere-pac fuel as a pressing raw material has been researched.
In case of uranium dioxide or a mixed oxide containing uranium as a base, a high density of pellet as 95% T.D. was obtained by 1982 [Literature 2]. Herein, % T.D. is relative density for theoretical density.
On the other hand, in case of thorium oxide or a mixed oxide containing thorium as a base, a high density of pellet was not obtained in that time because a dry gel particle obtained by the sol-gel method is in the glass state, hard and wrong in press characteristics [Literature 1 and 4) .
In case of this thorium-base oxide, a high density of pellet could not be obtained until the press characteristics has been improved in the joint work of Germany and Brasil (1979-1983), by mixing finely divided powder of carbon and poly vinylalcohol in the sol and making the particle soft by oxidatively removing carbon by high temperature heat treatment of gel particle (in this case, as a sintering aid, calcium was previously added in the sol.) [Literature 5]. Thereafter it has been published 1986 that this method is successful even in the condition of not using polyvinyl alcohol [Literature 3].
The above literatures are as shown in the following Table 1.
TABLE I ______________________________________ Literature 1: ORNL/TM-6906(1979) Literature 2: Trans. Am. Nucl. Soc., 40(1982)52-54 Literature 3: Nucl. Technol., 73(1986)84-95 Literature 4: J. Nucl. Mater., 92(1980)207-216 Literature 5: JUL-SPEZ-266(1984)86-121 Literature 6: J. Nucl. Sci. Technol., 25(1988)848-856 ______________________________________